Brain hexokinase exists in both cytoplasmic and mitochondrial forms. Furthermore, the mitochondrial form, both in vitro and in vivo, exists in equilibrium between soluble and particulate forms. The soluble-particulate distribution is influenced by certain metabolites, with glucose-6-P apparently being the most significant in vivo; variation in soluble-particulate distribution occurs concomitantly with changes in cerebral glycolytic rate and is thought to be involved in regulation of hexokinase activiy. The objectives of the proposed research will include: 1) determination of the relationship between ligand-induced conformational changes and regulation of the catalytic activity and soluble-particulate distribution of the enzyme, 2) determination at the molecular level (i.e. the groups directly involved) of the nature of the interaction which permits the selective binding of brain hexokinase by the outer mitochondrial membrane, 3) development of electron microscopic methods for assessing the soluble-particulate distribution of hexokinase and the rate of glucose utilization in defined neural structures (e.g., cells and nerve endings), 4) determination of the molecular basis for a recently discovered microheterogeneity of brain hexokinase, its origin and possible biological significance, 5) isolation of the cytoplasmic form of the enzyme and elucidation of the chemical differences between cytoplasmic and mitochondrial forms, their origin and biological significance, 6) determination of the relative levels of cytoplasmic and mitochondrial forms in various neural cell types, and correlation with other mitochondrial and cytoplasmic enzymes involved in cerebral energy metabolism.